The invention relates to a machine element of a metallic material, particularly a piston and piston rod, for a shock absorber for motor vehicles, with at least one hard-material coating deposited on the machine element.
In hydraulics and pneumatics, the piston of a shock absorber is a machine element that is subjected to particularly high mechanical stresses. Seals are intended to prevent the leakage of hydraulic oil from the chamber in which the piston moves. To reduce wear and prevent corrosion of the piston, electrodeposited hard chromium coatings are used, among others.
Despite the pronounced hardness of chromium coatings, the service life of components so coated is regarded as inadequate. Moreover, the corrosion protection is occasionally considered not fully satisfactory. Also a low coefficient of sliding friction is desired. Finally, the stringent requirements on environmentally benign disposal of the electrolytic baths necessary for chromizing pose a special problem.
It is further known that hard, amorphous, hydrogen-containing carbon (a--C:H) coatings, either in the "pure" form or with admixtures of other elements, have low coefficients of sliding friction.
Finally, the deposition of silicon and carbon-containing coatings from organosilicon compounds by means of a glow discharge is also known.
The copending U.S. patent application Ser. No. 271,176 discloses a sealing element consisting of a ceramic sealing member around which water circulates, and having a platelike, pistonlike or spherical shape, with a hard-material coating which renders the sealing element extremely wear-resistant and assures, moreover, that the sealing element slides easily on its seat without the use of a separate lubricant, the sliding motions being relatively slow.